Chelsea have been hit by a shattering blow after midfielder Michael Essien suffered a serious knee injury, which threatens to rule him out for the season.

The 25-year-old damaged his anterior cruciate ligament playing for Ghana against Libya on Friday and must now undergo an operation once the swelling has gone down.

Essien's agent Fabien Piveteau said: "I am afraid it is a big problem. He's taken a bad blow to the knee.

"It is now two or three times that he has taken blows to his knee."

Chelsea would not put a date on his return but admitted it would be "several months" before he can play again.

With the transfer window closed until January, Essien's absence limits Luiz Felipe Scolari's options in midfield - Michael Ballack is already sidelined by an ankle problem - as the club bid to win back the title from Manchester United.

A Chelsea spokesman said: "Michael has started rehab in the aqua therapy centre at Cobham which will improve the condition of the knee prior to surgery. A return to full fitness is not expected for several months."

Without Essien, John Obi Mikel is likely to face Manchester City on Saturday.

The midfielder, who missed the last two League games with a leg injury, said: "It is a lot better. The past two weeks I have been working very hard."

Didier Drogba could also play at City as he nears full fitness following a knee injury.